Forming dies for the repetitive stamping of large numbers of identical sheet metal parts are made of high strength tool steel because of its rigidity and durability. In the automotive industry, forming dies made of tool steel are used to stamp automobile body parts from steel sheets. However, such dies are expensive and require appreciable manufacturing time because large castings must be made and complex forming surfaces accurately machined.
It is a common practice in the automotive industry that before a new vehicle having a new body style is put in production, prototype vehicles are first built for testing. Designing forming tools with tool steel for stamping sheet metal parts used in these prototype vehicles would not be practical for several reasons. First, a prototype vehicle has to be built in a relatively short time which prohibits the use of tool steel for stamping dies due to the extensive machining required. Secondly, the design of a prototype vehicle is frequently changed many times from its original design before it reaches a final production model. This means that many stamping dies would have to be built before the design of a sheet metal part is finalized, thus making the building of stamping dies with tool steel prohibitive for cost reasons.
Prototype stamping tools have been made using a zinc alloy material called Kirksite. Even though a Kirksite tool is more economical to make than a steel die, a Kirksite tool still requires the use of a foundry as well as extensive machining to obtain the desired contour and close match between the tool halves. Furthermore, Kirksite tools are heavy and difficult to handle.
In recent years, there has been a renewed effort to develop mass-castable polymeric materials to make large and durable sheet metal forming tools. One family of these commonly used materials is epoxy resins.
It is well known in the art that it is extremely difficult to formulate a mass-castable, room temperature, fast curing epoxy that can be cast-to-size into high strength tools. By mass-castable, we mean the pouring of a liquid epoxy composition in one casting step to produce a completed portion of a tool. By cast-to-size, we mean a process in which the exact contour is obtained on the surface of a tool such that no machining, barbering or spotting on the tool surface is required.
For instance, U.S. Pat. No. 4,423,094 to Dearlove et al disclosed a tough, durable bisphenol A epoxy composition for use in making sheet metal stamping dies. While this material exhibits good mechanical strength, it does not form a durable punch for stamping tools. Moreover, it requires an extensive curing procedure, i.e., it must be hardened at room temperature overnight and then post-cured at 150.degree. C. for two hours. Since most stamping tools for automobile body panels are large in size and weight, it is preferable to make dies that need no elevated temperature curing.
Others have used room temperature-curable epoxy resin systems such as those based on bisphenol A and an aromatic amine catalyst to make plastic tools. An extensive curing period of from four days to one week is required for this type of room temperature-curable epoxy composition. To achieve rapid cure, i.e., to cure in less than 24 hours, an aliphatic amine catalyst instead of an aromatic amine catalyst must be used in an epoxy molding composition. This type of rapid curing epoxy system has been used in adhesives and coatings where the maximum thickness of the epoxy layer is no more than 6.4 mm thick.
Attempts to use unfilled aliphatic amine catalyzed epoxy compositions in bulk casting epoxy tools met with little success. The major problems encountered in casting a bulk section epoxy tool with a rapid curing epoxy composition were severe shrinkage and dimensional distortion which led to unacceptable warpage of the tool. These problems were caused by the extreme heat generated by the exothermic curing process. Since the curing process proceeds very rapidly, the exothermic heat accumulated in a bulk section epoxy tooling could not be rapidly dissipated by heat transfer due to the low thermal conductivity of epoxy. This caused the formation of localized heat pockets and thermal shocks which led to shrinkage, cracking, and dimensional distortions.
When a material is selected for building stamping tools, both its compressive strength and its tensile strength are important considerations. To sustain a high compressive load in the vertical direction, a tooling material must have high compressive strength. Similarly, to sustain a high tensile load in the horizontal direction, a tooling material must also have high tensile strength. This type of tensile load, for example, is frequently seen in the cavity of a stamping tool having a V-shaped concave configuration in the tool surface. No commercially available mass-cast epoxy tooling materials have the necessary combination of compressive strength and tensile strength for making stamping tools.
U.S. Pat. No. 4,920,161 to Wang et al disclosed that within a specific range of formulations, 24-hour room temperature cured cast-to-size epoxy tooling materials with high tensile strength can be obtained. These formulations contained high loadings of specific particle sizes of inert fillers, in particular, silicon carbide and silica. The patent specifically taught rapidly-curable, distortion-free compositions that achieved high tensile strength as compared to commercial tooling materials.
It is desirable to use inert fillers other than silicon carbide or silica in epoxy tooling compositions. Tools cast with silicon carbide and/or silica fillers were found to be very difficult to subject to post processing such as minor machining, drilling, tapping, etc., for incorporating engineering changes. In addition, silicon carbide is moderately expensive and adds to the cost of the tooling material. However, substitution of other known suitable particulate fillers for the silicon carbide resulted in significant reductions in tensile strength.
It is, therefore, an object of the present invention to provide a mass-castable, highly filled epoxy tooling composition that has high tensile and compressive strength sufficient for making durable stamping tools that can be machined to make post-processing changes.
It is another object of the present invention to provide an epoxy tooling composition that can be rapidly cured at room temperature in less than 24 hours without significant dimensional distortion.
It is yet another object of the present invention to provide an epoxy tooling composition that can be rapidly cured at room temperature in less than 24 hours to make a cast-to-size metal sheet forming die by utilizing inert fillers other than silicon carbide.
It is a further object of the present invention to provide an epoxy tooling composition that can be rapidly cured into a dimensionally stable machinable sheet metal forming tool by utilizing a suitable combination of appropriately-sized iron fillers, short glass fiber reinforcements, and a surface active agent.